The present invention relates to high surface area electrodes, and to a process for the preparation of electrodes having high surface area.
Prior art electrodes include those fabricated from metal plates, roughened and etched metal plates and conductive filler particles compounded in an insulative binder. These prior art electrodes are limited in surface area and therefore provide a significant energy loss on passage of current. Further, in many applications such as the delivery of energy into a liquid medium, gas generation is undesirable. Such applications include the electrodes used in some hot water heating systems and further, those used in heart pacemaker and defibrillator systems and those used in membrane bio-sensors. The high surface area electrodes of the present invention substantially reduces such gas generation.
U.S. Pat. No. 4,687,598 (Varma) describes the preparation of an electrode-active material for batteries wherein the material consists of an electrochemical material in contact with a ligand and a binder. This electrode-active material is not capable of providing a high surface area electrode having high capacitance, low polarization, low energy loss and low gas generation on the passage of current through the electrode.
U.S. Pat. No. 4,551,267 describes the preparation of a plastic electrical conductor for battery electrode applications. However, because this conductor utilizes a plastic binder material which has only a very low ionic conductivity, this conductor cannot serve as a high surface area electrode. Only a small part of the surface area of its carbon or graphite particles, namely those resident at or near the surface of the electrode will operate as part of the electrochemical interface.
French Patent 2,442,514 disclose a composite electrode material consisting of a mass of electrically conductive material dispersed within a mass of a solid solution of an ionic material dissolved in a plastic macro molecular material. This composite electrode material is restricted in use because of the need to provide a dissolved ionic compound. Such a dissolved ionic compound would leach out at an electrode made from such a material if placed in an aqueous solution. This would occur, for example, in hot water heating and pacemaker electrode application of these electrodes.
In order to avoid the loss of the ionic material in such applications, it is necessary to provide ionic groups bound to the polymeric backbone as described in the present invention. The materials described in French Patent 2,442,514 would also tend to swell and disintegrate in an aqueous media. The present invention provides materials which have a controlled degree of swelling and therefore remain intact during use in aqueous media.
A further advantage of the electrodes described herein in applications requiring delivery of energy into the ion containing medium is the avoidance of electrochemical reactions at the interface. For example, where the ion containing medium is a water based solution, operation of prior art electrodes commonly produces hydrogen at the cathode and oxygen at the anode. Such gas production is generally undesirable. The electrodes of the invention avoid such gas generation by virtue of their high surface area and therefore their lower over potential during operation at a given current.
A still further advantage of the electrodes described herein is their ability to be molded into any desired shape.